Node Metadata: Add Infotext explaination

chapters/node_metadata.md

@@ -13,6 +13,7 @@ In this chapter you will learn how to manipulate a node's metadata.
 * Reading Metadata
 * Setting Metadata
 * Lua Tables
+* Infotext
 * Your Turn
 
 ## What is Node Metadata?
@@ -107,6 +108,16 @@ tmp.foo = "bar"
 meta:from_table(tmp)
 {% endhighlight %}
 
+## Infotext
+
+The Minetest Engine reads the field `infotext` in order to make text
+appear on mouse-over. This is used by furnaces to show progress and signs
+to show their text.
+
+{% highlight lua %}
+meta:set_string("infotext", "Here is some text that will appear on mouse over!")
+{% endhighlight %}
+
 ## Your Turn
 
 * Make a block which disappears after it has been punched 5 times.